function fearNotLetter(str){
    // 将字符串转化为ASCII码
    let arr = [];
    for(let i =0 ;i<str.length ; i++){
        arr.push(str.charCodeAt(i))
    }
    console.log(arr);
    for(let j=1;j<arr.length;j++){
        let num = arr[j] - arr[j-1];
        if(num != 1){
            return String.fromCharCode(arr[j]-1)
        }
    }
    return undefined;
}

console.log(fearNotLetter('abce'))